1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and, more specifically, to a manufacturing method of a nonvolatile semiconductor memory device including a variable resistive element having a variable resistor made of a perovskite-type metal oxide film.
2. Description of the Related Art
In recent years, a variety of device structures such as an FeRAM (Ferroelectric RAM), an MRAM (Magnetic RAM) and an OUM (Ovonic Unified Memory) have been proposed as a next generation nonvolatile random access memory (NVRAM) that makes a rapid operation possible substituting a flash memory, and there has been a fierce competition for the development of such devices from the standpoint of increase in performance, increase in reliability, cost reduction and process matching. However, these memory devices at the present stage have their merits and demerits, and there is a long way to go before the realization of an ideal “universal memory” which has the respective merits of an SRAM, a DRAM and a flash memory.
The FeRAM, for example, which has already been put into practice, utilizes a spontaneous polarization inversion phenomenon of oxide ferroelectrics and is characterized by a low consumed power and a high speed operation; however, it is inferior due to high cost and destructive readout. A ferromagnetic tunnel effect element that utilizes giant magnetoresistance (GMR) used in the MRAM has a structure in that an extremely thin insulating layers (tunnel barrier layers) such as Al2O3 is placed between two ferromagnetic layers made of Fe, Co, Ni or the like. Herein, an amount of tunnel current flowing via the insulating layers can be controlled by changing the orientation of magnetization (spin) of the ferromagnetic layers, thus exhibiting a memory effect. This element has a large problem of high consumed power for magnetization inversion at the time of programming and of a difficulty in miniaturization. In addition, the OUM based on thermal phase transformation of a chalcogenide material is advantageous in low cost and process matching; however, it has a problem in miniaturization and in a high speed operation due to its thermal operation.
In addition to these existing technologies, a resistive random access memory (RRAM) device, that utilizes an electrical pulse induced resistance (EPIR) effect which is a new phenomenon in a colossal magnetoresistance (CMR) material, has been disclosed (see U.S. Pat. No. 6,204,139) by Shangquing Liu, Alex Ignatiev et al. of Houston University in the United States. The EPIR effect in a CMR material represented by a Mn-based oxide material having a perovskite-type structure is epoch-making where a change in resistance by several digits occurs at room temperature. The RRAM that utilizes this phenomenon has features in a low consumed power, a simple structure appropriate for miniaturization, easiness in a high integration and a wide dynamic range of a change in the resistance, and has excellent properties where a multiple value memory, which stores information of three or more values in a single memory element, is made possible. The memory element has an extremely simple basic structure where a lower electrode thin film, a CMR thin film and an upper electrode thin film are layered in sequence in the direction perpendicular to a substrate. According to the operation, the polarity, the voltage and the pulse width (widely ranging from several tens of ns to several μs) of the electrical pulse that is applied between the upper and the lower electrodes are controlled, so that the resistance of the CMR thin film that is placed between the upper and lower electrodes is changed. The resistance value that has been changed due to such a pulse application is maintained for a long period of time after the pulse application, and the performance of a nonvolatile memory element can be obtained by, for example, making the low resistance condition correspond to “O” and making the high resistance condition correspond to “1”.
As the CMR material of an EPIR element, Pr1-xCaxMnO3 (PCMO), La1-xCaxMnO3, La1-xSrxMnO3, Gd0.7Ca0.3BaCo2O5+5 or the like, which have a perovskite structure, where the base is a network of oxygen octagons having a 3d transition metal element at its center, are typically used, and there has been reported that PCMO having the composition that is close to x=0.3 has the widest range of a change in the resistance value. As the electrode material, metal-based materials such as Pt, Ir, Ru, Ph, Ag, Au, Al and Ta and oxide- or nitride-based compounds such as YBa2Cu3O7-x, RuO2, IrO2, SrRuO3, TaSiN, TiN, TiSiN and MoN which have a conductivity higher than the CMR material are used and noble metal-based materials which are superior in mass production and form an excellent interface condition with the CMR layer, causing no problems with the electrical connection and which include metals in the platinum group such as Pt (lattice constant a=0.3923 nm), Ir (a=0.3839 nm), Rh (a=0.3803 nm) and Pd (a=0.389 nm) as well as Au (a=0.4079 nm) are appropriate.
The present inventors have diligently continued research on nonvolatile memory elements that utilize pulse electric field induced resistance variation of the perovskite-type metal oxide films. As a result, the present inventors have found that some elements perform a switching operation in the perovskite-type metal oxide film while others don't, and that the range of the initial resistance values of the elements which perform the switching operation is limited to a specific range. FIG. 1 shows this range of the initial resistance values.
The element used for obtaining the range of the initial resistance values in FIG. 1 includes a Pt lower electrode, a PCMO film (50 μm×50 μm, film thickness: 100 nm) and a Pt upper electrode, which are layered in sequence. The resistance value is measured through calculation from current values measured when 0.8 V is applied to the upper electrode. The voltage pulse applied to the upper electrode has a pulse width of 100 ns and a pulse amplitude of 2 V.
The determination of whether or not the variable resistance elements that have performed a switching operation are good is made by confirming reproducibility of the resistance variation in a manner that the ratio of variation in the resistance value is no less than 3 and the resistance value varies from a low resistance to a high resistance, again to a low resistance and again to a high resistance, in a pattern which is repeated no less than four times, at the time when pulses having the two polarities, positive and negative, are alternately applied in the manner +2V, −2V, +2V, −2V . . . Here, the ratio of variation in the resistance value is in a range from 1.3 to 1.5 for an MRAM, which is one of nonvolatile memory elements that have been developed in recent years. The conditions for determination of the ratio of variation in the resistance value in the PCMO film are stricter.
It has been found out from FIG. 1 that the initial resistance values of all of the measured elements are in a range from 1 kΩ to 1 GΩ, while the initial resistance values of the elements that have performed the switching operation are limited to a range from 4 kΩ to 2 MΩ. Though the range of the initial resistance values of the elements that perform the switching operation is from 4 kΩ to 2 MΩ in the example shown in FIG. 1, it has been confirmed that the range of the initial resistance values when performing the switching operation varies, depending on the pulse application voltage, the pulse width, the composition of the PCMO film, and the conditions for formation. Accordingly, it is necessary to control the initial resistance value of a variable resistance element to its appropriate value for operation of a nonvolatile semiconductor memory device, in order to obtain a variable resistance element that performs the switching operation.